i'll just try to answer the other questions you have as i don't have much weight in what i can say about thickness of spines. in my opinion, it really is more about the grind of the knife than the thickness. just so long as you get a good thickness on the spine, and a great grind you're pretty close to what people would want.
4) Is a convex geometry good for a kitchen knife?
convex is awesome. with japanese knives they convex one side (generally the right side for righties) and leave the opposite side (left) generally flat. this creates a problem for lefties, like myself, coz when they make knives, the food you cut sticks on the left side and makes the knife steer (depends on how the knife is ground) because of that assymetric grind and edge. the japanese do usually a steeper angle on the left and a wider angle on the right that's around 70/30 most of the time when they sharpen so that too complicates things for lefties. some j makers also do a 60/40, 80/20, 90/10, but that depends on the application and what kind of knife is made.
some even do a 50/50 bevel (but some still say it's pretty much comes out as 60/40) but still sharpens it to a 70/30 edge, which is okay for lefties coz we can just sharpen out that edge and make it more usable for us.
it really is up to you as you are the maker. =D
5) Any other tips for a new maker?
folks here love a french chef's knife profile like a sabatier or even old vintage american profiles like forgecraft and ontario cutlery. so you might wanna get some inpiration off of them.
oh and we hate finger guards like the ones on sabs. some folks just grind that off when they buy vintage. messes with the sharpening process. we also love rounded spines (at the very least the part where a person would normally do a pinch grip) and choils.
another couple things
we love our carbon steels as much as stainless or even probably more so. depends on who you talk to. and we like higher hardness on our knives like 60RC and up. as for handles, it's a very complicated subject so you'll just have to come up with one the general public would like. just see what kinda handles on here we like on the knives we post about and see what you would wanna make.
=D
of course, everything i've said is just opinion and probably just a general consensus. you're still the maker, it still ultimately falls to you how YOUR knife will look and perform.
maybe when you make a prototype you'd wanna do a passaround and see what they think to further refine it. nothing like actual hands on experience.
i'll shut up now. =D